The invention is related to an actuator, comprising a housing which contains a motor and a screw mechanism, said screw mechanism comprising a screw and a nut one of which is rotatably supported with respect to the housing, and a gear reduction mechanism connecting the rotor of the motor to the rotatable screw or nut.
Such actuator is generally known, and can be applied for various purposes such as for actuating a brake, a clutch etcetera. In many of these applications, it is desirable to have a large reduction between the motor and the object to be actuated, such as brake pads. On the other had, overall dimensions and weight should remain limited.
The screw mechanism itself provides a reduction which i.a. depends on the pitch of the threads. A small pitch however entails rather narrow thread, which in particular for ball screws would lead to small, vulnerable balls which moreover are difficult to handle.
The gear reduction means therefore preferably should provide a relatively large reduction, such that the screw mechanism can have coarser threads which provide a lower reduction of said screw mechanism itself.
In this respect, the prior art gear reduction mechanisms are not adequate. They comprises a ring gear, satellite wheels and a crown gear. Usually, two or more of these mechanisms are arranged in series so as to obtain the required reduction. The overall dimensions and weight are high.
The object of the invention is therefore to provide an actuator having a gear reduction means with a large reduction, and with relatively small dimensions and low weight. This object is achieved in that the gear reduction mechanism comprises at least a concentric gear ring with radially inwardly directed teeth, an excentrically positioned gear wheel having radially outwardly directed teeth wherein the outer diameter of the gear wheel is smaller than the inner diameter of the gear ring, such that the teeth of said gear wheel and gear ring engage each other along a part of their circumferences, and at an opposite part of their circumferences are out of engagement, said gear wheel being rotatable accommodated on a extentric hub which is connected to the rotor of the motor.
The reduction gear means according to the invention provides a large reduction as a result of the extentric position of the gear wheel and hub. Its dimensions arm small in thickness direction, leading to a compact embodiment.
Preferably, the gear ring is integrated with the screw of the screw mechanism, said screw being rotatably supported with relation to the housing.
Furthermore, a very compact actuator is obtained in case the rotor of the motor is rotatably supported on the outer ring of a support bearing, said outer ring being integrated with the screw and the gear ring.
By means of a radially inwardly extending flange, the rotor may be connected to the excentric hub.
Also, a positive back-drive mechanism is connected to the flange and the housing.
The actuator may be assembled from several sub assemblies such as a housing module, an actuator module and a gear reduction module.
According to a further embodiment, the housing hay have a bore accommodating at least the nut and/or the screw, and an axially fixed part of said nut or screw, supported with respect to a radial support abutment, which extends inwardly in the bore. In this embodiment, the rotor of the motor is supported rotatably on a sleeve, said sleeve engaging the fixed part and extending away from an actuator head which is movable in axial direction by means of the nut or the screw, said sleeve having a radially outwardly extending sleeve flange which is interposed between said support abutment, and the axially fixed part.
In particular, the flange of the sleeve is supported on an abutment surface of the support abutment, which faces the actuating head connected to the axially displaceable nut or screw for exerting a compressive force.
The rotor sleeve is now held firmly clamped between the radial support abutment and the axially fixed part of the nut or screw, which provides a simple, reliable construction. No additional fastening means are necessary for supporting the rotor sleeve.
In a practical embodiment, the nut is fixedly supported within the housing, said nut having a radially outwardly extending nut flange facing the outwardly extending sleeve flange and overlapping the inwardly extending actuator support abutment.
In service of the screw actuator according to the invention, misalignment may occur as a result of eccentric forces and/or transverse forces. Such misalignment might cause damage to the balls and raceways of the nut and the screw, which are rather vulnerable to such loadings. According to the invention, this problem can be alleviated in case the outwardly facing surfaces of sleeve flange and the nut flange are curved in axial cross section, so as to allow swivelling or tilting of said nut and sleeve due to misalignment forces.
The screw/nut unit, as well as the drive unit including rotor and possible reduction means, may now move in unison so as to adapt to the misalignment.
A very stable embodiment is obtained in case the nut has a nut extension extending beyond the nut flange and inside the support abutment, the sleeve having an axially extending support part which is accommodated between said nut extension and the support abutment.
According to a preferred embodiment, the actuating head engages the screw through a rolling element bearing. Said rolling element bearing is integrated with the actuating head. A further improvement concerning stability is obtained in case the actuating head is accommodated in a cylinder, which is held non-rotatably in the bore.
The sleeve may comprise a sheet metal part which is provided with the inner reaceway of at least one support bearing for rotatably supporting the rotor; alternatively, a separate bearing may be mounted with its inner ring on said sleeve.
For ease of handling and assembly, the sleeve and the nut are clampingly preassembled.
The sleeve may furthermore comprise an inwardly extending flange at its end opposite the sleeve flange, which flange carries a gear wheel of the gear reduction mechanism.